1. Field of the Invention
The present invention relates to an apparatus for reproducing video, audio and other signals that are recorded in optical recording media such as an optical disk. More particularly, the invention relates to an optical pickup which is used to reproduce information from a multi-layer recording medium including alternating half-mirror type and full-mirror type reflective recording layers and which is capable of attenuating the optical noise due to reflection from layers other than where information is being reproduced.
2. Description of the Related Art
A practice recently adopted to increase the recording capacity per surface of an optical disk such as DVD (digital versatile disk) includes forming two recording layers by evaporating a half-mirror type reflective film on the top and a full-mirror type reflective film on the bottom. In a playback mode, a light beam is converged on either one of the information bearing surfaces of the recording layers and the reflected light is detected with a detector, whereupon the signal recorded on that particular information bearing surface is reproduced.
A problem with this method of converging a light beam on one information be aring surfaces is that the reflected light from the other information bearing surface is out of focus when it overlaps the reflected light from said one information bearing surface, thus producing playback noise.
If more than two recording layers are placed in superposition with a view to increasing the recording density, more light is reflected from information bearing surfaces other than where the incident light beam is converged and this further contributes to increased noise. Consider, for example, an optical disk having three equally spaced information bearing surfaces. If a light beam is converged on the information bearing surface which is the remotest from a light source, the light reflected from the intermediate surface will also be converged just on the surface nearest to the light source. Since the light beam converging on the nearest surface has been modulated to high frequency, it will generate particularly great noise.
The present invention has been accomplished under these circumstances and has as an object providing an optical pickup for reproducing information from a multi-layer recording medium which uses optics for separating the reflected light from one information bearing surface and that from another information bearing surface, thereby ensuring that the crosstalk from information bearing surfaces other than where information is to be recorded is sufficiently attenuated.
The stated object of the invention can be attained by the optical pickup according to a first aspect of the invention which reproduces information by applying a light beam onto one of information recording surfaces formed in a multi-layer recording medium, said optical pickup including a separating means for separating the reflected light from said one information recording surface and the reflected light from another information recording surface which converges in a different position than the first mentioned reflected light and a detector for receiving the reflected light from said one information recording surface as separated by said separating means, the detection output from said detector being used to generate a playback signal.
Thus, the reflected light from the information bearing surface where information is to be reproduced is sufficiently separated from the reflected light from any other information bearing surfaces that a satisfactory playback signal can be obtained that is free of the xe2x80x9ccrosstalkxe2x80x9d component, or the undesired signal component of the reflected light that is contained in the signal component of the reflected light from the information bearing surface where information is being reproduced.
A second aspect of the invention is a modification of the optical pickup according to the first aspect of the invention, wherein said separating means is an astigmatic device that introduces astigmatism into the reflected light from said another information recording surface, said detector being located on the focal line of the reflected light from said another information recording surface.
Thus, the signal component of the reflected light from information bearing surfaces other than the surface where information is to be reproduced is sufficiently converged on the focal line that only the signal component from the information bearing surface where information is being reproduced can be selectively extracted using a simple structural design.
A third aspect of the invention is another modification of the optical pickup according to the first aspect of the invention, which further includes a converging position transforming device by means of which the reflected light from said one information recording surface is allowed to converge on the same plane as the reflected light from said another information recording surface, said detector being located in the position of convergence.
Thus, a converging position transforming device typically composed of a hologram device is used to separate the reflected light from the information bearing surface where information is to be reproduced and the reflected light from other information bearing surfaces and this ensures that only the signal component from the information bearing surface where information is being reproduced and which contains a reduced amount of the signal component from other surfaces is extracted with a simple structural design.